1. Field of the Invention
The present invention relates to a mobile communication system which permits independent setting of the transmission rates of up- and down-links.
2. Description of the Related Art
Mobile communication systems have a structure in which subscriber stations, i.e. mobile stations (such as vehicle-installed communication devices and portable communication devices) are connected to a base station via radio channels.
In these mobile communication systems, there is the possibility that sharing of radio frequency spectra (hereinafter referred to as “frequency sharing”) by different radio systems is implemented by an FDMA/TDMA combination, while in the CDMA system frequency sharing between different codes or symbols has already been put into practice. Handover in these systems is already known.
For example, time slot sharing mobile communication system which shares a TDMA signal and a time division CDMA signal in the same time slot is disclosed in our U.S. Pat. No. 5,805,581′ (Japanese Pat. Appln. Laid-Open Gazette No. 8-130766, EP701337A).
For instance, in the case of building a multimedia in such a mobile communication system as mentioned above, it is necessary that the channel capacity of an up-link over which the subscriber station sends data to the base station and the channel capacity of a down-link over which the base station sends data to the subscriber station be set independently of each other. Moreover, to cope with variations in the amount of data sent during communication, it is necessary to establish a system which enables the data transmission rate to be changed accordingly.
For example, upon occurrence of a situation in which a momentary increase takes place in the amount of data to be sent from the subscriber station to the base station, the subscriber station computes the transmission rate required taking into consideration the amount of increase in the data to be sent, and sends a request for a change of the transmission rate to the base station.
Upon receiving the request for a change of the transmission rate from the subscriber station, the base station determines whether a change of the transmission rate can be accepted after hunting for an idle channel in the link over which they are currently in communication with each other.
And, when judging that a change of the transmission rate can be accepted, the base station sends to the subscriber station the identification number of the communication channel to be used thereafter.
Upon receiving from the base station the identification number of the communication channel for subsequent use from the base station, the subscriber station sends data over the designated communication channel to the base station after that.
In this way, the data transmission rate is changed, but since the actual change of the data transmission rate calls for a process of obtaining permission from the distant or remote station, there is a need to allow for a considerable amount of waiting time until the transmission rate is actually changed after the amount of data to be sent changes.
Accordingly, in a mobile communication system in which the amount of data sent undergoes great variations and allowance for an appreciable amount of waiting time is difficult to make, it is customary to adopt a scheme which fixes the transmission rate at a large value by assigning a wide frequency band from the beginning of communication.
Since the conventional mobile communication system has such a configuration as mentioned above, the transmission rate can be changed when permission is obtained from the distant station after sending thereto a request for a change of the transmission rate, but a considerable amount of time is needed until the transmission rate is actually changed. Hence, when the amount of data to be sent varies greatly, congestion of data or the like occurs, leading to, for example, impairment of the immediacy of data sent in real time.